How to make object slightly thinner?
Here's how I did it (and I think this is the cleanest method):

0. Original object



1. Loop cut the original object twice

As the object has a hole, the Loop Cut and Slide (Ctrl+R) tool doesn't loop cut both the internal and external sides together.  A better way is to use the Extrude Region (E) tool.

Rotate view totally horizontal, and with the face select mode enabled, select all the bottom faces:



Use the Extrude Region tool to extrude the bottom faces by 1 mm (this will be the height of the bevel at the bottom).  Use the Extrude Region tool again to extrude by another 1 mm (this value is arbitrary, and will be removed later):



2. Reduce the thickness of the object

Select all the side faces:



Using the Shrink/Fatten (Alt+S) tool, shrink the object by 0.2 mm (this is the tolerance of the object).  Hold down the Alt key while pressing the Enter key to enable the Even Thickness option.  This is the result:



3. Make the bevel at the bottom

Select the side faces nearest to the bottom of the object:



Use the Shrink/Fatten (Alt+S) tool to shrink the faces by 1 mm (the width of the bevel).  Again, hold down the Alt key while pressing the Enter key to enable the Even Thickness option.  This is the result:



4. Remove the the part of the object below the bevel

Select the bottom faces:



Translate the faces upwards by the arbitrary amount chosen in Step 1 (in this case, it is 1 mm):



Switch to vertex mode, and re-select all the bottom vertices:



Use the remove doubles tool to remove the double vertices:



5. Reduce the extra height from the bevel

Select the vertices at the bottom two layers (encompassing the whole bevel region):



Translate the vertices upward by the height of the bevel (1 mm in this case):



Done!